docs: def pl2nvc(plane):
      """
      Return a unit normal vector and constant that define a specified plane.
  
      http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/pl2nvc_c.html
  
      :param plane: A SPICE plane.
      :type plane: supporttypes.Plane
      :return:
              A normal vector and constant defining
              the geometric plane represented by plane.
      :rtype: tuple
      """
      assert (isinstance(plane, stypes.Plane))
      normal = stypes.emptyDoubleVector(3)
      constant = ctypes.c_double()
      libspice.pl2nvc_c(ctypes.byref(plane), normal, ctypes.byref(constant))
      return stypes.cVectorToPython(normal), constant.value